The attitude or angular orientation of a satellite or other spacecraft may be controlled by exchanging angular momentum between the craft and an array of control moment gyros (CMGs). The CMGs must have extremely low torque jitter which is an extremely difficult design constraint for mechanical bearings. A primary cause of torque jitter in existing CMGs is that mechanical bearings, because of direct contact between the rotating and stationary elements, transmit vibrations arising from rotor unbalance to the supporting structure.
The use of magnetic bearings in CMGs has the primary advantage that physical contact between the rotor and stator is eliminated. Vibration and structural interaction can be reduced through the use of active control. Magnetic bearings have been demonstrated for use in CMGs for "small" satellites. The word "small" refers to the magnitude of the torques which must be applied to the spacecraft (up to several hundred Newton-meters). For "large" spacecraft (those which require torques as large as several tens of thousands of Newton-meters), flux saturation in conventional core materials requires either that a large, heavy magnetic structure be employed, or that the magnetic solenoid consume a great deal of electrical power. Either alternative is undesirable in a spacecraft. In addition, magnetic bearings typically gimbal the flywheel about a relatively small angle of only 10-20 degrees, which limits the amount of angular momentum transferable between the flywheel and the spacecraft.